Some methods are available for determining the refractive index. In these methods, the refractive index may be measured using ellipsometry, or by determining the critical angle of the total reflection, or internal reflection, at or off of layers, or by employing other methods based on the refraction of the light.
When working with the smallest material quantities, whose structural dimensions vary within the micrometer or nanometer range, the available methods for determining refractive indices can fail. For example, because of insufficiently concentrated exciting radiation and improper detection of the scattered or deflected radiation, existing methods are not suitable for amounts of matter constituted as rods, whose diameter is less than 50 micrometers. Particular difficulties arise when the matter only exists in quantities whose dimensions are less than the wavelength. This is true of certain types of material, such as photonic crystals, which can be fabricated using additive nanolithography methods, including corpuscular beam-induced deposition.